Electron discharge device



Filed Nov. 4, 1953 FIG.|

FIG. 2

H o .rzwmmau P5 50 I00 I50 200 250 INVENTOR PLATE V L RICHARD T. ORTH.

BY wfl aor ATTORNEY Patented May 12, 1936 UNITED STATES ELECTRON DISCHARGE nsvrca Richard T. Orth, Orange, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application November 4, 1933, Serial No. 696,610

4 Claims. (01. 250-275) My invention relates to electron discharge devices and more particularly to radio tubes of the pentode type having an outer electrode which surrounds the plate and acts as a screen.

The screen grid or tetrode tube of the type disclosed in U. S. patent to Mitchell, 1,748,026, issued February 18, 1930, usually has an outer mesh screen which surrounds the tubular plate and is electrically connected to the inner screen grid inside the plate through a top metal disc or washer over the end of the plate. The inner screen and outer mesh screen operate at a positive potential, which is usually intermediate the cathode and plate potentials. The average plate current characteristics of a screen grid tube are apt to show some instability when the plate voltage drops down to or below the screen grid voltage, hence operation of the plate at screen grid voltages, although sometimes desirable as from a power consumption standpoint, cannot be recommended. This instability is believed to be due at least in part to secondary electron emission from the plate, the outer mesh screen, and the connecting top shield.

One explanation of the secondary electron emission is that when the plate voltage is higher than the screen voltage, some of the primary electrons strike the inner screen grid and others strike the plate with suflicient force to cause emission of secondary electrons from the screen grid and from the plate. The secondary electrons from the screen grid go to the more positive plate and produce an increase in plate current, which decreases the plate impedance and the output of the tube. When the screen grid voltage is higher than the plate voltage, the secondary electrons from the plate will be attracted to the screen, and this loss of secondary electrons by the plate causes an undesirable decrease in the plate current. An additional or suppressor grid electrode interposed between the inner screen grid and the plate, and maintained at a lower potentiaLsuch as ground potential, will substantially prevent the flow of secondary electrons between the inner screen and the plate, but will not prevent the effects of secondary electrons arising from the outer mesh screen and from the top shield, both of which are positive.

One of the objects 01' this invention is to provide a pentode type of electron discharge tube which will operate satisfactorily when the plate voltage is substantially the same or less than the screen grid voltage.-

Another object of this invention is to provide a pentode type electron discharge tube with an outside screening electrode substantially free of secondary electron emission.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, will best be understood by reference to the following description taken in connection with the accompanying drawing in which:

Figure 1 is a perspective view with parts broken away of a pentode type tube made in accordance with my invention.

Figure 2 shows curves of the average plate current characteristics of the ordinary screen grid or tetrode tube and of a pentode tube, one curve being that of a screen grid tube with outer and inner screen grids operating at positive potential, and the other two curves of a pentode type of tube, one when the outer screen is operated at screen grid potential, and the other when it is operated at cathode potential.

The tube shown in Figure 1 comprises a highly evacuated envelope ill with the usual press ll having the usual inner leads I! for the electrodes enclosed by the bulb. The electrode assembly comprises an electron emitting thermionic cathode l3, a control grid II, a screen grid IS, a suppressor grid IS, a tubular plate l1, and an outer screen comprising a top metal disc or shield l8 over the end of the plate and an outer mesh cage is surrounding the plate. The control grid, screen grid, and suppressor grid, the plate, and the outer screen are all tubular and coaxial with the cathode and are positioned successively around it in the order named.

The control grid I4 and screen grid l5 operate in the usual way, and the suppressor grid l6, connected to its own terminal or base pin, is operated at a potential lower than the lowest potential of the plate, which causes it to act as a suppressor grid. As the suppressor grid l6 may to advantage be operated at ground or cathode potential, it may be connected internally to the cathode I3 if desired.

The top shield I 8 and the outer screen I 9 are carried by and electrically connected to support rods 20 extending from the press II. A bottom shield or metal collar 2i carried on the rods 20 is connected to the cathode I3 through a cathode tab 22, so that the outer screen IS, the top shield I8, the bottom shield 2|, and the cathode I3 are all electrically connected and operate at cathode potential, which may conveniently be applied through a terminal connected to one of the rods 20.

some of the beneficial results produced by having the outer screen and top shield connected to operate a cathode potential instead of at positive screen grid potential are such increased stability of the average plate current characteristics at low plate voltages, that operation with plate voltages about the same as the screen voltage is feasible; increased plate impedance of the tube with corresponding increase in the output of the tube; decrease in screen grid current; and sharper cut-oil.

The curves 23 and 24 in Figure 2 show the relation between variations of plate voltage and corresponding changes in average plate current in the usual screen grid tube or tetrode and the usual pentode tube, when the other electrode voltages, such as the control grid and screen grid voltage, are those generally used in normal operation of the tubes. Curve 23 shows the plate current characteristic of .the usual screen grid tetrode type of tube with the top shield and outside screen operating at screen grid potential. The effects of secondary emission from the plate, the inner screen grid, the top shield, and the outside screen are evident from the variations in plate current at difierent plate voltages, for example, the marked drop in plate current at a plate voltage of about 50 volts. Curve 24 shows that in a pentode type of tube having a suppressor grid at ground potential, and the top shield and outside screen at screen grid potential, variations in the plate voltage below approximately volts cause substantial changes in the plate current. Curve 25 shows that in a similar pentode in which, in accordance with my invention, the suppressor grid, top shield, and outside screen are all at ground potential, the same plate voltage variation causes only very slight changes in the plate current. Such a tube made in accordance with my invention can be operated at comparatively low plate voltage because the usual fluctuations in plate potential during operation of the tube do not produce the same effects as are shown by curve 24. The screen grid and the plate may be operated at about the same potential, thereby eliminating voltage dividers and similar equipment necessary when the screen and plate must be operated at difieront potentials.

While I do notwish to be restricted to any particular theory, it is believed that in the usual screen grid or tetrode tube there is, in addition to secondary emission from the screen grid or plate, some secondary electron emission from the outer mesh screen around the plate and from the top shield over the end of the plate, due to stray primary electrons from the cathode escaping around the ends of the grids and plate, striking the positive top shield and outside screen, and producing secondary emission which lowers the plate impedance and decreases the output of the tube. If, in accordance with my invention, the outer screen and top shield are electrically connected to the cathode, as shown in Figure 1, these two shielding members will not attract the primary electrons which escape around the ends of the grids and plate and no secondary emission occurs from them. Furthermore, any flow of secondary electrons from the inner walls of the glass bulb I0 due to the impact of primary electrons escaping the plate I! and screen [9 will be suppressed by the screen H! at cathode potential.

While I have shown and described the preferred embodiment of my invention, it will be apparent that many modifications may be made in the structural details employed for carrying my invention into efiect without departing from the scope thereof, as set forth in the accompany.- ing claims.

I claim:

1. An electron discharge tube comprising a hermetically sealed glass envelope enclosing an electrode assembly which comprises a thermionic cathode and a cooperating tubular anode, a plurality of grid electrodes positioned between said cathode and said anode, a pair of spaced parallel metal rods independent of said grid electrodes and extending lengthwise through said anode, a pair of metal shields positioned on and in electrically conducting engagement with said spaced rods at opposite ends of said cathode and said anode, an electrical connection between said cathode and one of said shields, and a mesh screen electrode surrounding said anode and electrically connected to and supported by one of said shields.

2. An electron discharge device comprising a sealed envelope having a press at one end and enclosing an electrode assembly supported on said press and comprising a straight cylindrical thermionic cathode, a plurality of tubular grid electrodes coaxial with and surrounding said cathode, a tubular anode coaxial with and surrounding said grids, a pair of spaced parallel metal support rods extending from said press lengthwise through said anode and independent of said grid electrodes and of said anode, an annular metal collar mounted on said rods at the end of said assembly adjacent said press, a metal tab on said cathode electrically connected to said collar, a metal disc shield larger than said anode and mounted on said rods remote from the press to cover and shield the other end of said assembly, and a mesh screen electrode secured to and depending from the edgev of said disc shield to surround said anode.

3. An electron discharge tube comprising a sealed envelope enclosing an electrode assembly comprising a straight indirectly heated cathode, a tubular anode coaxial with and surrounding said cathode, a plurality of tubular grid electrodes coaxial with said cathode and interposed between said cathode and said anode, a pair of spaced parallel metal rods extending through said anode between said anode and said grid electrodes, a disc shield positioned on said rods to extend over the top of said anode and transversely of the longitudinal axis of said anode, and a tubular mesh screen electrode surrounding said anode and electrically connected to and supported solely by said disc shield.

4. An electron discharge device comprising an oblong sealed envelope enclosing an electrode assembly comprising a straight thermionic cathode extending longitudina-lly of said envelope, a tubular anode coaxial with and surrounding said cathode, a plurality of tubular grid electrodes of difierent diameters mounted coaxially with and surrounding said cathode and between said cathode and said anode, a support rod longer than said anode and grid electrodes and extending parallel to said cathode through said anode and independent of said grid electrodes and of said anode, a metal connector inside said envelope electrically connecting said cathode to said support rod, a metal disc shield mounted on said support rod to extend transversely of said envelope and to project beyond the sides of said anode, and a tubular mesh screen electrode electrically connected to and supported by said metal disc and projecting from said disc to be coaxial with and to surround said anode.

. RICHARD 'I. ORTH. 

